Assessing the non-target effects of herbicides on field margin plant communities after controlling for soil, climate, local context and landscape metrics

Pesticides are often identified as one of the major causes of biodiversity decline in farmlands. However, our knowledge about this relationship has mostly being inferred from small to landscape-scale studies, or from indirect indicators of agricultural practices at large scales. Here, we used a national network of more than 500 sites monitored yearly from 2013 to 2018 in France to assess the non-target effects of herbicides on field margin plant communities. We used hierarchical generalized linear models to investigate the effects of practices on plant species richness, plant species evenness, proportion of nature-value plants, and proportion of grasses in field margins, while controlling for a large number of possible confounding effects. The intensity of herbicide use had a negative effect on plant species richness, and on the proportion of nature-value plants. In the margin of cereal fields, there was a negative effect of dicotyledon herbicides on richness and a negative effect of grass herbicides on species evenness. We also identified, in some specific crops, a negative effect of non-herbicide treatments on margin flora richness and on the proportion of nature-value plants. The presence of surrounding grasslands had a consistent favourable effect on richness and on the proportion of nature-value plants in field margins. Finally, situations of risk for pesticides drift had a negative effect on margin flora. This study confirms that reducing herbicide use represents a robust lever to maintain the floristic diversity of field margins, which could be combined with strategies reducing the risk of pesticide drift. • We used a national dataset of 500 sites monitored yearly from 2013 to 2018. • We analysed the effects of herbicides on plant margin communities. • Herbicides had a negative effect on richness and nature-value species. • Situations of risk for pesticides drift had a negative effect on margin flora.